The present invention relates to environmentally stable Mach-Zehnder devices and to methods of making such devices.
There is an emerging need for narrowband wavelength division multiplex (WDM) couplers and filters. Such devices are needed in the 1550 nm window for modifying the gain spectrum of erbium fiber amplifiers. They will also be widely used in trunk lines as well as in fiber-to-the-subscriber architectures.
There is a need for both wavelength tunable components and fixed wavelength components. In an all optical network, for example, the device can be tuned at the receiver end in order to detect the desired incoming signal. In a second approach, tunable lasers are used to send a plurality of signals, and the desired signal is detected by employing a receiver having a fixed filter. The transmission system could also employ both fixed lasers and filters. The wavelength separation capabilities of such filters needs to be on the order of tens of nanometers to as small as a nanometer. Moreover, these components will need to be environmentally stable and very reliable.
The Mach-Zehnder filter is known for its narrowband wavelength capabilities. It has been proposed that filters having pass bands as narrow as 1 nm be formed by connecting two evanescent couplers with unequal fiber lengths between them. See OFC Conference on Optical Fiber Communication, Minitutorial Sessions, Jan. 22-26, 1990, page 256 (part of a presentation on "Dense WDM Techniques" by C. A. Brackett). It is difficult to achieve reproducibility and environmental stability with this approach, since the connecting fibers are subject to external destabilizing conditions such as temperature changes and random bending forces.